Many modern automatic transmissions use trim regulator valves, solenoid valves, and accumulators to control the apply pressure rise for an on-coming torque transmitting device such as a clutch or brake. The trim regulator valve controls the apply pressure gain from a value sufficient to overcome the spring force exerted by the clutch return springs to full engagement pressure. The maximum full engagement pressure occurs during vehicle operation requiring maximum engine torque and in some instances, maximum engine torque multiplied by a torque ratio of a torque converter, which can be as high as 3 to 1.
The trim pressure control at the lower end of the range is very important. During this portion of the clutch apply, it is necessary to overcome the return spring force and initiate frictional engagement. If the apply pressure rise is too rapid, a harsh shift feel may result.
Additionally, once the clutch is filled with hydraulic fluid pressure, an overshoot condition may occur if there is a lack of compliance within the hydraulic clutch control system, or if the solenoid valve is slow to react. Lower cost solenoid valves tend to exhibit slow reaction times. One way to eliminate the pressure overshoot condition is to add a continuous leak to the clutch feed passage. This leak will add additional compliance to the hydraulic clutch control system and, if designed properly, may eliminate pressure overshoots. This continuous leak is present at all times during clutch trim and full apply pressures. Additionally, some clutch designs employ additional hardware within the clutch such as compliance or wave plates. The plates are a mechanical means for introducing additional compliance within the hydraulic clutch control system during clutch apply conditions.